Update, internally, consists of following steps (simplified) : 1. read record 2. evaluate new record 3. write new record into data page
really, it is a more complex, but for now we interesting only in steps above. At step 1, read-committed (RC) transaction waits for commit or rollback of concurrent active transaction and thus reads only comitted record version. At step 3, transaction re-reads original record version and must ensure that it is still the same record version as it was at step 1 (else we will silently replace other's work). If current record version was changed since step 1, update_conflict error will be reported - exactly as you see. Again, it is simplified description. Note, time slice between steps 3 and 1 usually very small. When you test concurrent updates in isql, you can't reproduce such scenario. But in real life, when you have a lot of concurrent updates by applications it becomes much more probably and really happens sometimes. Hope it helps, Vlad
